Ozone gas is an oxidizing agent that is used to treat polyethylene resins. The free radicals generated by the ozone can abstract hydrogen from the polyethylene backbone. Depending on the reaction conditions, e.g. temperature, the removal of hydrogen can result in chain scission, chain extension, crosslinking or the addition of functional groups to the polyethylene resin.
One approach of modifying polyethylene is to oxidize the polyethylene resin at a temperature above its melting point (Tm). Oxidizing the polyethylene resin can result in significant reductions in molecular weight and increases in melt indices (MI). It can also cause crosslinking and increasing in viscosity. The increase in viscosity can prevent the rapid diffusion of ozone through the molten polyethylene. Moreover, the reduction in molecular weight can limit the applications that the polyethylene resin is used for, see GB Pat. No. 1,087,914.
Solid-state oxidation of the polyethylene resin is another approach. GB Pat. No. 997,135 discloses oxidizing a polyethylene resin at a temperature ranging from 90° C. to 5° C. below the melt temperature (Tm) for a period of time ranging from 12 hours to 36 hours. An oxygen containing gas is used and optionally a promoter, e.g. ozone, in an amount ranging from about 0.05 wt. % to about 5 wt. %, see also GB Pat. No. 951,308. This approach also causes significant reduction in molecular weight.
There exists a need for new methods of modifying polyethylene resins that would enhance the properties of the polyethylene without the drawbacks, e.g. significant reductions in molecular weight.